(0) Stop. Don't panic. Take a deep breath. Look around. This problem is new and mystifying right now but it's actually not terribly serious; certainly not terribly uncommon. Your data is probably perfectly safe. But you need
to clearly establish what
the situation is, and what
you are going to do about it, before
you proceed. Get out a pencil and a legal-pad and start making notes. Force
yourself to stop,
and don't panic.
(1) Boot to a LiveCD. This gets you into a Linux environment where you can "look around."
(2) Read carefully and completely: info grub
. Do not neglect
this. Do not simply "read as little as possible and then rush off and try it." Your computer will wait.
(3) The thing that will determine which boot-loader gets run is the master boot record (MBR),
which the normal Grub install process will write for you. The BIOS searches for MBRs and runs the first one that it finds. The MBR, in turn, runs the boot loader. Notice that the BIOS can search several
devices, as stipulated in its "Setup" screen. (Go look
at that, and write down
what you find on your legal-pad.)
needs to find a /grub
directory on the boot partition and certain files in it. When it finds them, it can display a nice menu. It also has a set of commands that you can use if the menu is, say,
(5) There are specific instructions about how to boot Windows from Grub.